A conventional wire bonding apparatus 100 is depicted in FIG. 1. The apparatus includes an electrical flame-off (EFO) wand 104, a bonding wire 108, which is fed through a wire damper 112 and a capillary 116. The wire damper 112 and the capillary 116 are parts of what is typically referred to as a bond head assembly (the bond head assembly may include other components such as, for example, an ultrasonic transducer). A ball 120 is formed at the end of the wire 108 that protrudes from the narrow end of the capillary 116. The apparatus 100 also includes a lead frame 122 having leads 124 and 128. A semiconductor integrated circuit or die 132 is mounted on the lead frame 122 and forms a die-lead frame assembly. The die 132 includes bond pads 136 and 140. A wire 144 is bonded to the pad 140 and the lead 124. The apparatus 100 also includes a lead frame holder 148, which secures the die-lead frame assembly during wire bonding, and an electrical discharge generator 152 coupled to the wire damper 112 and the EFO wand 104. The electrical discharge generator creates an electrical discharge in response to receiving a discharge signal.
Bonding the wire 108 to the die 140 and the lead frame 122 is accomplished using the bond head. The bond head generally moves in the x, y, and z directions above the die-lead frame assembly.
Conventionally, the ball 120 on the end of the wire 108 is formed by placing the electrical flame-off (EFO) wand 104 a predetermined distance from the end of the wire 108. An electrical arc is emitted between the end of the wire 108 and the EFO wand 104. The arc forms the ball 120 on the end of the spool of bonding wire 108. By varying the intensity and the duration of the electrical arc, the size of the ball that is formed can be adjusted to specific dimensions.
FIG. 2 depicts a prior art EFO wand 104, which includes a mounting section 164, epoxy 168, and an electrode 172. The mounting section 164 is both rigid and electrically conductive. The epoxy 168 is electrically conductive and secures the electrode 172 to the mounting section 164. As discussed above, the electrode 172 emits an electrical discharge or arc from a tip 176 to the end of a bonding wire 108 to form a ball 120. The electrode 172 is formed from a rigid and electrically conductive material. Typical electrodes 172 are made of platinum having a purity of 99.95%. The diameter of a typical electrode is 0.020 inches.
It has been determined that, during use, a certain amount of carbon contamination forms on the EFO electrode each time a bonding ball 120 is formed. The carbon contamination acts as an insulator. As such, build up of carbon contamination interferes with the normal functioning of the electrode tip 176. As the carbon contaminates the electrode, it gradually degrades the quality of the bonding balls that are formed. A variety of other contaminants also build up on the wand and interfere with the ball formation process in a similar manner.
Mechanical scraping of the electrode tip 176 is one possible method of removing the contamination which has been deposited. However, this method has serious drawbacks in that continual mechanical adjustments would be required to avoid excessive damage to electrode tip 176. Additionally, a mechanical apparatus for scraping the electrode tip would desirably be located on the automated ball bonding machinery near the normal operating position of the electrode 172, a location where space is already at a premium. Therefore, it would be desirable to provide an improved method or design for cleaning an electrode of an EFO wand.